1. Technical Field
The present invention relates to a VGS turbocharger used in an engine for a motor vehicle or the like, and more particularly to a novel manufacturing method which can eliminate a cutting operation and a welding operation as much as possible, at the time of manufacturing an adjustable blade to be incorporated in the turbocharger.
2. Description of Related Art
A turbocharger is known as a supercharger used as means for improving the power output and the performance of an automobile engine. The turbocharger is an apparatus in which a turbine is driven by the exhaust energy of the engine to rotate a compressor with the power of the turbine, whereby the engine is supercharged to have more air fed into it than fed into it by natural suction. In the case of an engine capable of running at up to a high rotational speed region, when the engine is running at low rotational speeds, the exhaust turbine of the turbocharger hardly functions due to the reduced flow rate of the exhaust gas, so that the engine can not avoid giving a slow-moving feeling until the exhaust turbine runs efficiently, and necessitating a subsequent time or a so-called turbo-lag before the turbine rapidly reaches the full-running state. Furthermore, in the case of a diesel engine which runs inherently at low rotational speeds, there is a disadvantage in that it is difficult to produce an effect of the turbocharger.
Therefore, a VGS turbocharger that works efficiently even when the engine is running at low rotational speeds has been developed. The turbocharger of this type is adapted to obtain a high power output when the engine is running at low rotational speeds by throttling flow of exhaust gas at a low flow rate with a plurality of adjustable blades (vanes) disposed on an outer periphery of the exhaust turbine to increase the velocity of the exhaust gas and increase work of an exhaust turbine. For this reason, in the VGS turbocharger, an adjusting mechanism for the adjustable blades is required additionally, and it is required that the associated constituting parts be formed to have a complicated shape or the like in comparison with those of the conventional one.
It has been common that, when an adjustable blade for such a VGS turbocharger is manufactured, a metal material (or a shaped material having a starting form for the adjustable blade) including a blade portion and a shaft portion that are integrally formed is first formed, for example, in accordance with a precision casting method represented by a lost wax casting method, a metal injection molding method or the like, and the shaped material is then suitably subjected to cutting or the like, to thereby finish the blade portion and the shaft portion to have desired shapes and dimensions.
However, in the technique using cutting operations applied to the shaped material, the following problems arise. Since this kind of turbo apparatus is constructed so as to introduce exhaust gas and utilize energy of the exhaust gas, a surface member thereof is naturally exposed to a high-temperature atmosphere of the exhaust gas. Since the exhaust gas contains constituents that can corrode the metal material, a heat resisting stainless steel such as SUS310S having excellent heat, oxidation resistance and the like is also used for the adjustable blade. However, since such a material is generally difficult to cut and therefore requires a lot of time to cut it, there is a problem that a lot of trouble is taken for processing it. In addition, since about ten to fifteen adjustable blades are required in one turbocharger, it is necessary to manufacture 300,000 to 450,000 adjustable blades a month in the case that about 30,000 motor vehicles are actually mass-produced monthly, so that the cutting operation is incapable of coping with this (production of about 500 adjustable blades is a limit in the cutting operation).
In view of the foregoing, there has been developed a method of manufacturing an adjustable blade from which the cutting operation is excluded, and the method is disclosed, for example, in Japanese Patent Application Publication No. 2000-145470 “Adjustable vane applied to adjustable vane type turbocharger and manufacturing method of the same”. In this Japanese Patent Application Publication No. 2000-145470, there is disclosed a method of manufacturing an adjustable blade, in which a blade portion and a shaft portion are separately formed, followed by welding them. A reasonable evaluation is obtained by this method in view of the fact that no cutting operation is required, however, there leaves room for further development.
In other words, in the method of manufacturing the adjustable blade by welding the previously separately formed blade portion and shaft portion, it is extremely difficult to weld the blade portion and the shaft portion while keeping a fixed angle and to accurately achieve a height of the vane corresponding to a width of the blade in a stable state, and thus variations are generated. Further, the shapes of the blade portion and the shaft portion in the respective single part states can be achieved accurately, however, the desired dimensional accuracy can not be obtained in the final product due to an adverse effect applied by a welding heat and a welding trail (a welding bead). Accordingly, there has not been achieved a stage that the adjustable blade can be mass produced at a stable and high quality level. Originally, as the adjustable blade is constructed such as to rotate around the shaft portion so as to throttle the exhaust gas by the blade portion, the height of the blade, the mounting angle of the blade portion and the shaft portion and the like are important dimensional values greatly relating to the performance of the turbocharger itself, and a high degree of accuracy is strongly required.
Additionally, recent years have seen tighter regulation of exhaust gas emission into the atmosphere particularly from diesel vehicles from a viewpoint of environmental protection and the like. For a diesel engine which inherently runs at low rotational speeds, mass production of a VGS turbocharger capable of improving the engine efficiency from a low rotational speed region has been strongly desired in order to reduce NOx, particulate matter (PM) and the like.